Keratin is a strong protein that makes up our hair, nails, skin and teeth. Humans are not the only ones who produce keratin, most animals do as well. Due to the cysteine disulfide, keratin is almost non-dissolvable. Depending on the concentration of cysteine disulfide, the bond can create extremely hard cells such as hooves, teeth, and horns or the cells can become softer as in hair and skin. There are high levels of cysteine disulfide (a sulfur-containing amino acid) which give a distinct sulfurous odor when the keratin is burned.